1. Field of the Invention
The present invention relates to discrete point liquid level sensing within an enclosure, and more particularly to a rapid-response liquid level sensing system that includes two temperature sensing probes and a circuit that produces an output proportional to the time rate of change of the difference between the two signals received from the temperature sensing probes.
2. Description of Related Art
For aerospace launch vehicle applications using liquid propellant, precisely determining when a fuel tank has been depleted during flight is critically important to ensure proper engine cutoff. Typically, several sensors within a fuel tank of the launch vehicle monitor the liquid level in the fuel tank as the fuel exits the tank. The sensors function to prevent the engines of the launch vehicle from shutting down either too early or too late. An early shutdown could prevent the launch vehicle from reaching orbit; a late shutdown could result in an engine failure.
Conventional liquid level sensing systems for launch vehicle applications use a bare wire sensing element that adequately meets system response requirements (<160 ms), but is very fragile. The bare wire sensing element consists of a thin platinum wire wrapped loosely around a ceramic card. The wire diameter is extremely thin—about the width of a human hair. The thermal mass of the thin wire is very small, which means that the response time of the sensing element is fast because liquid is quickly shed from the sensing element during the transition from liquid to gas. However, the fragility of the conventional sensing element is problematic, especially for newer launch vehicles that experience significantly higher pyrotechnic shock levels. In addition, at cryogenic temperatures, the low resistance (˜1 ohm) of the conventional liquid level sensors interferes with circuit fault detection.
There is a compelling need for a liquid level sensing system that provides sufficiently fast response times with higher resistance, while being rugged enough to endure high mechanical shock levels. The present invention addresses this need.